Install pg for development

Changed the development and test databases from Sqlite to Postgres.
I don't know if there are general Ruby community guidelines. The big projects I've worked on put very explicit versions in, just to be really safe (no `~>` and certainly no `>=`). In my own projects, I sometimes leave them out entirely, because doesn't the Gemfile.lock take care of that? If you want to research this and let me know about any best practices, I'd be interested to learn.